[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <5522FE84.6030104@cchtml.com>
Date: Mon, 06 Apr 2015 16:45:40 -0500
From: Michael Cronenworth <mike@...tml.com>
To: netdev@...r.kernel.org
Subject: Re: e1000e max frame calculation
Preface: I am not here to start a discussion about header sizes or hardware limits.
This is solely about the calculation used to limit a user when they pass a MTU to a
driver.
On 04/06/2015 11:38 AM, Michael Cronenworth wrote:
> Other drivers appear to be using the "old" maximum frame calculation and can
> continue to use 9000.
>
> Can a standard be decided upon that all drivers use or is it acceptable that one
> driver "does its own thing?"
To be specific here are some examples of maximum MTU calculation and their tests:
sky2: user_mtu > ETH_JUMBO_MTU
r8169: user_mtu < ETH_ZLEN || user_mtu > (ETH_DATA_LEN||x*1024 - ETH_HLEN - 2)
igb: user_mtu + ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN > MAX_JUMBO_FRAME_SIZE
e1000e <3.15: user_mtu + ETH_HLEN + ETH_FCS_LEN > adapter->max_hw_frame_size;
e1000e >=3.15: user_mtu + VLAN_HLEN + ETH_HLEN + ETH_FCS_LEN >
adapter->max_hw_frame_size;
Let's say the hardware has a limit of 4096 bytes. The user wishes to set an MTU of
4078 bytes.
sky2: false, mtu is set
r8169: false, mtu is set
igb: true, mtu is not set
e1000e <3.15: false, mtu is set
e1000e <3.15: true, mtu is not set
So some drivers work with the 4078 MTU and some do not. They all had the same
hardware limit, but their driver's software calculated limit was different. Am I crazy?
Thanks,
Michael
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists